There are various types of power that can be generated from the motion of oceans, rivers and wind. One type is called Ocean Thermal Energy Conversion (OTEC). OTEC uses temperature differences at various ocean depths to generate power using any of a number of power generation techniques. OTEC is, in general, very inefficient (2-3% total efficiency) and costly compared to other types of ocean power generation schemes, and there are no known commercial power generation systems at present.
Another type of ocean energy uses ocean waves to generate power. In general, ocean wave energy systems are more expensive relative to conventional power plants, and they have significantly variable output from day to day.
Other sources of ocean energy are from large currents, such as the Gulf Stream, from rivers, from tidal currents and from off-shore wind, and are discussed in more detail below.
The generation of energy from major ocean currents, such as the Gulf Stream, appears quite feasible, although there are questions regarding how the installation of power turbines may affect the current and the environment. If one were to tap into the Gulf Stream where it comes closest to shore in South Florida, one could potentially capture perhaps as much as 5% of the total potential energy in the fastest portions of the flow. This would amount to about 1510 MW (see equation 1 below), or enough to power about 1.5 million homes in Florida.Power=0.5*rho*velocity3*area,  (Eq. 1)whererho=1025 kg/m3velocity=1.7 m/secarea=100-m×30,000-m
The other source of ocean current energy is from ocean tides, or “tidal energy”. Tidal energy is generated by the relative motion of the Earth, Sun, and the Moon, which interact via gravitational forces. Due to these, gravitational forces, water levels follow periodic highs and lows. Associated with these water level changes, there are tidal currents. The specific tidal motion produced at a certain location is the result of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth, Earth rotation and the local shape of the sea floor
One means of generating power using the tides is to construct barrage dams to trap the tide and then use hydroelectric power generators to electricity. There are three known systems worldwide, which use this type of power generation. They are located in France's Rance River, Canada's Bay of Fundy, and Russia's Kislaya Guba. These systems are considered environmentally damaging, however, in that they create silt buildups behind the dam, and they impair and damage the natural flow or marine life.
The other means of generating power is to use in-stream tidal generators, which make use of the kinetic energy from the moving water currents to power turbines, usually in a similar way that wind mills use moving air. This method is gaining in popularity because of low cost and low ecological impact. The first two tidal generators actually attached to a commercial power grid are in Hammerfast, Norway and in New York City's East River. Both of these systems use a windmill type of arrangement that is attached to the ground.
A description of the internal working of a tidal turbine (10) is shown in FIG. 1. The water current slowly turns a large turbine blade (20). The rotation is increased through a gearbox (30), and is converted to electricity by means of a rotating generator (40). Also shown in FIG. 1 are a rotor (50), seals and bearings (60), and a stationary housing (70). The power is then usually conditioned to the same voltage and phase as other turbines in the same water current, and the power lines (80) are buried until they reach the shore, where they are connected to a commercial power grid.
The primary problems with this type of system are that it is costly to fabricate and bury the underwater power lines, and the electronics are necessarily subject to corrosion. This is due to the fact that the external water, often seawater, can leak in through rotating seals, and rapidly corrode the copper electronics, as shown by arrow (90).
Another type of tidal turbine is produced by an Australian company, Tidal Energy Pty Ltd. The system uses a venturi turbine that is enclosed in a housing to produce what is claimed to be an efficiency of 3.85 times that of the conventional tidal turbine system. This system also suffers from, the same problems of costly buried electrical cables, as well as salt-water intrusion into subsurface electronics.
A third type of tidal energy system is being developed by a British company, Lunar Energy. This system uses tidal turbine blades to turn a hydraulic oil pump, which delivers high pressure oil to a hydraulic motor/generator located above the shrouded blades.
As shown in FIG. 2, some components are similar from the prior art embodiment of FIG. 1. However, differently from FIG. 1, FIG. 2 shows a fluid pump (100) that pumps low pressure hydraulic liquid (110) to become a higher pressure liquid (120), which activates a hydraulic motor (130) to turn a generator (140), which is immediately adjacent to and submerged with the pump. The electricity then travels to shore by means of a buried power line (150).
This design shown in FIG. 2 is more costly to build, but it greatly minimizes the chance of seawater getting inside of the generator, since the hydraulic liquid is at a higher pressure than the surrounding see, and thus would tend to leak out into the surrounding seawater. It still suffers, however, from the expense of buried electrical cables.
Another type of tidal turbine from Marine Current Turbines (not shown) uses twin propellers that are mechanically attached to an above-surface power turbine. This is basically similar to the conventional tidal turbine, except that two rotors are used to counter each other's torque, and there is the added expense of building, potentially very tall towers that must withstand ocean storms.